We have discovered and begun the characterization of a protein called Nun, which appears able to promote both termination and antitermination of transcription. Nun, encoded by the Lambda-related bacteriophage HK022, promotes transcription termination on a Lambda template, but appears to suppress termination on an KH022 template. Suppression of termination by Nun appears to be analogous to the action of several previously characterized bacteriophage antitermination factors such as the phage Lambda N protein: the location of the nun gene in HK022, between a major early promoter (pL) and a transcription terminator, corresponds to that of the N gene in Lambda. We have also shown that transcription termination by Nun on a Lambda template requires the same protein (Nus factors) and sites (nut sites) that are required for N-promoted antitermination on the same template. These findings suggest a fundamental unity in the mechanisms of termination and antitermination. The frequency of int-promoted recombination between two phage Lambda attachment sites is reduced when the nucleotide sequence of one of the sites differs from that of the other within a 7 bp segment called the overlap region. Recombination is thought to proceed through a branched DNA intermediate called a Holiday structure that is resolved to recombinant products. We have shown that synthetic Holliday structures that contain one copy of an overlap region mutation called safG and one copy of a wild type overlap region are not converted to recombinant molecules by int protein, but instead are converted back to parental molecules. Work in another laboratory has shown that safG nonhomology does not prevent the formation of Holliday structures by int protein. Together, these findings argue against a model in which direct interaction between homologous nucleotides of the overlap regions is required for synapsis. We now favor a model in which such homologous interaction is required for resolution of the recombination intermediate to recombinant products. A simple molecular mechanism for such an interaction is branch migration of the Holliday structure from one end of the overlap region to the other.